1. Field of the Invention
The invention relates to a method and to a device for producing a screen printing stencil by rotating a screen printing cylinder to irradiate a light responsive layer thereon.
2. Description of Related Art
It has already been disclosed in DE 36 01 327 A1 to make use for the purpose of forming a thin screen printing stencil, in particular for textile printing, of a screen printing cylinder which is, for example, produced galvanoplastically, is extremely thin-walled, is made from, for example, pure nickel and has a multiplicity of small passage openings. This uniformly perforated screen printing cylinder bears on its outer surface a thin lacquer layer which seals its passage openings. In order to obtain a desired printing pattern, the surface of the screen printing cylinder is partially exposed in a dedicated operation, specifically by removing the lacquer layer situated on it. In this operation, use is made of a laser which either burns off or evaporates the lacquer at the points to be exposed, or initially crosslinks or polymerizes by intensive exposure a photosensitive or photopolymerizable lacquer of a different type. There are also lacquers whose cohesive power is so far damaged by the intensive effect of light that they can be removed from the exposed points by means of a subsequent chemical after-treatment.
The laser beam is focussed onto the lacquer layer and switched on and off very rapidly, often up to a few million times per second, it being simultaneously displaced relative to the lacquer surface, and so the total result of exposing the small focal spots is to produce the desired pattern of the screen printing stencil.
Normally, the screen printing stencil rotates relatively quickly, for example at 1,500 revolutions per minute, it being the case that the focal spot of the laser beam which impinges at least approximately radially onto the screen printing cylinder is simultaneously displaced in the axial direction of the screen printing cylinder, with the result that the focal spot on the surface of the screen printing cylinder moves along a helix. The lead of the helix corresponds approximately to the diameter of the focal spot, or is slightly smaller. The entire process is controlled by a fast computer, which is respectively aware of the circumferential and axial positions of the focal spot through interrogating appropriate sensors, with the result that the individual exposures are correctly added to an overall pattern.
The screen printing cylinder bearing the lacquer layer on its surface is pushed for the purpose of producing a pattern onto an expandable, exactly concentric clamping cylinder, but this is relatively complicated. If, in addition, screen printing cylinders of different diameter are to be processed, it is also necessary in the case of relatively large deviations in diameter to change the relatively heavy and unwieldy clamping cylinder.
Furthermore, DE 29 02 902 A1 has disclosed the production of a screen printing stencil by applying to a metallic cylindrical screen coated with a photosensitive photo-resist a film to which the photo-resist is exposed. The metallic cylindrical screen is supported at both its end faces by centering flanges which form, together with the cylindrical screen, a pressure chamber into which a pressure medium is introduced in order to clamp the cylindrical screen by applying pressure. Thereafter, the already mentioned film is laid onto the outer surface of the cylindrical screen thus treated, and exposed. The reason for clamping the cylindrical screen is to increase its natural stability, which normally is not sufficient when surface processing is undertaken on the outside of the cylinder, for example mechanical or photographic designing. A relatively high dimensional stability is required for such work.